The method of using a hook, like a crochet hook, to pull up on the latch and free the door does work. It takes some fiddling to figure out exactly what piece you need to hook on and pull up. The Canonet has a pretty narrow hole for the rewind crank, so you may not easily get a crochet hook in there, it will need to be one of the smallest sizes. A bent piece of metal like from a bobby pin might work (are they called bobby pins in the UK? Clips to hold long hair in place.)
This is a pretty common problem, so if you can find a shop that knows about film cameras, as Brian says they can probably fix it quickly. When you get it out, make sure to screw the parts back together tightly. Use something in the slot of the rewind fork to hold it while you tighten the knob.